The present invention relates to the structure of a polymer electrolyte fuel cell. More particularly, the present invention relates to a stack assembly structure of a membrane electrode assembly (MEA), gaskets, and electrically conductive separators.
The conventional polymer electrolyte fuel cell has a basic power generation element as described below (see Japanese Patent No. 3045316). FIG. 21 is a plan view showing the structure of an assembly comprising a polymer electrolyte membrane electrode assembly which is the conventional basic power generation element and a gasket 106. The polymer electrolyte membrane electrode assembly is called an MEA (membrane electrode assembly). FIG. 22 is an enlarged plan view showing the structure of the portion represented by XXII in FIG. 21. FIG. 23 is a cross-sectional view taken along line XXIII-XXIII in FIG. 22.
Referring to FIG. 23, an MEA 15 comprises a polymer electrolyte membrane 12 formed by an ion-permeable membrane that selectively passes hydrogen ions, a pair of catalyst layers (cathode catalyst layer 13 and anode catalyst layer 14) disposed to interpose the membrane 12 between them and containing carbon powders carrying a platinum-group metal catalyst as a major component, and a pair of gas diffusion electrodes 107 provided on outer surfaces of the pair of catalyst layers 13 and 14, the gas diffusion electrodes having an outer periphery located inwardly relative to the outer periphery of the polymer electrolyte membrane 12. The gas diffusion electrodes 107 are formed chiefly by carbon fibers having both gas permeability and electron conductivity. In order to inhibit a fuel gas or an oxidizing gas to be supplied to the gas diffusion electrodes 107 from leaking to the outside and to inhibit these gases from being mixed, a pair of gaskets 106 are provided on peripheral portions of surfaces on both sides of the MEA 15, so as to have gaps 109 between the gaskets 106 and the gas diffusion electrodes 107. The MEA 15 and the gaskets 106 are joined to each other typically by a thermo-compression bonding process. Hereinafter, the MEA 15 provided with the gaskets 106 is called an MEA-gasket assembly. Although illustrated to be enlarged in FIG. 23, a width of the gaps 109 between the gas diffusion electrodes 107 of the MEA 15 and the gaskets 106 is typically approximately 0.2 to 0.5 mm. The provision of the gaps 109 permits displacement between the gas diffusion electrodes 107 and the gaskets 106, thereby facilitating their assembly.
In addition to the above conventional structure, there has also been proposed an MEA integral with gaskets (see U.S. Pat. No. 5,464,700, and Japanese Laid-Open Patent Application Publications Nos. 2002-42838 and 2001-155745).
The basic principle of the above conventional polymer electrolyte fuel cell is such that one principal surface of the polymer electrolyte membrane 12 is exposed to the fuel gas and the other principal surface thereof is exposed to the oxidizing gas, such as air, so that chemical reaction occurs in the vicinity of the membrane 12 to generate water, and the resulting reaction energy is recovered as electricity.
However, in the conventional polymer electrolyte fuel cell, since the gaps 109 are provided between the gas diffusion electrodes 107 and the gaskets 106 as shown in FIGS. 21 and 22, part of the gases supplied to the interior of the fuel cell tend to be discharged through the gaps 109.
As shown in FIG. 21, a fuel gas supply manifold hole 3A and a fuel gas discharge manifold hole 3B are formed on opposing sides in a peripheral portion of the MEA-gasket assembly. One surface of the anode-side electrically conductive separator (see FIG. 4) provided with a fuel gas passage on a surface thereof is in contact with the principal surface (surface shown in FIG. 21) of the MEA-gasket assembly exposed to the fuel gas. As represented by broken lines in FIG. 21, the fuel gas supply manifold hole 3A and the fuel gas discharge manifold hole 3B of the MEA-gasket assembly (to be precise, fuel gas supply manifold hole and fuel gas discharge manifold hole of the electrically conductive separator) are connected to each other through the fuel gas passage of the separator. In this structure, the fuel gas passage and the gap 109 cross each other so as to fluidly communicate with each other. As indicated by the arrows in FIG. 21, a part of the gas flowing from the fuel gas supply manifold hole 3A into the fuel gas passage flows through the gap 109 and into the fuel gas discharge manifold hole 3B. The fuel gas flowing through the gap 109 is discharged without being exposed to the gas diffusion electrode 107. One surface of the cathode-side electrically conductive separator (see FIG. 3) provided with an oxidizing gas passage on a surface thereof is in contact with the principal surface of the MEA-gasket assembly exposed to the oxidizing gas. In the same manner as described above, a part of the gas flowing from the oxidizing gas supply manifold hole 5A into the oxidizing gas passage flows through the gap 109 and into the oxidizing gas discharge manifold hole 5B. The presence of cell reaction gases which are not exposed to the gas diffusion electrodes 107 reduces the utilization ratio of these gases and hence reduces power generation efficiency.
Therefore, the conventional polymer electrolyte fuel cell needs to have the utilization ratio of the cell reaction gases increased.
Meanwhile, using MEAs with integral gaskets (MEA-gasket assemblies), the fastening device for the cells must have a large size because of the need for a considerable force for fastening the cells of the fuel cell. In addition, since the gaskets are made from a material such as liquid EPDM or rubber which is highly elastic and highly durable with respect to temperature variation or reactive materials, the manufacturing cost of the fuel cell substantially increases.